Once more into the breach: New Jersey Gov. Jon Corzine has ratcheted up his efforts – waged by the state attorney general’s office and paid for with tax dollars – to keep secret his e-mail exchanges with ex-squeeze Carla Katz.

Assistant AG Patrick DeAlmeida has told a court it doesn’t have the legal right to order the governor’s staffers to explain what steps they took to find all the Corzine-Katz messages.

The state Republican Party had sued to make those e-mails public, and Superior Court Judge Paul Innes said he would examine the messages and decide whether they should be. DeAlmeida balked, and the GOP charged that Corzine aides were willfully disobeying the judge’s directive.

Corzine and Katz are more than just ex-lovers; their relationship ended with a secretly negotiated multimillion-dollar settlement. But the bigger complication is that Katz heads the state’s largest public-employee union.

Then it came out that the governor this year lavished extra cash on Katz’s brother-in-law – after the latter lost his state job when he was caught using it to spy on the tax records of Corzine’s political foes.

Apart from that, the governor and his ex can’t seem to keep their stories straight.

At first, they claimed the e-mails should remain secret because they were private messages. Then Katz claimed that they should stay hidden because they involved official labor negotiations – a claim the governor denies. (Moreover, her national union says she lacks the authority to conduct such discussions.)

But however complex this whole mess gets, it boils down to one simple point:

If everything about the Corzine-Katz e-mails is so innocent, why are they both expending so much effort to keep them secret?

People who have nothing to hide don’t spend so much time trying to hide it.